1. Field of the Invention
This invention relates to a control system for the testing, calibration and enhanced operation of industrial scales having multiple weight inputs.
2. Description of Related Art
Multiple input electronic scales have three basic elements; multiple load cells, a device referred to in the industry as a junction box, and a weight indicator. In its most simple form a load cell is constructed by sandwiching an electrical type strain gage between layers of rigid material. As weight is applied to the layers, current flow through the electrical strain gage varies in response to the amount of weight applied. A junction box receives input signals from the multiple load cells, sums and averages the signals and transmits the averaged signal to the weight indicator. The weight indicator receives the averaged signal from the junction box and electrically converts the signal to a readable form indicating the weight being applied to the scale.
More specifically, load cells are precision machined metallic devices which will deflect under load and return to normal state when unloaded. The normal rating of signal output on a load cell is stated as a millivolt signal, most typically 2 or 3 millivolts per volt of excitation. Typically excitation is 10 volts D.C. Therefore, a 3 millivolts per volt of excitation load cell being powered by a 10 volt D.C. power supply will output a 30 millivolt signal when loaded to its full rated capacity. For example, a load cell having a 1000 pound capacity will output 3 millivolts per volt of excitation or 30 millivolts of signal when loaded 100 percent,i.e., 1000 pounds. Likewise, the millivolt signal will be 15 millivolts at 500 pounds. The output is both proportional and linear to the weight applied to the scale.
When multiple load cells are used, for example, under the four legs of a tank, the voltage signals are summed in a junction box located at the base of the tank. The junction box is located in close proximity to the load cells to minimize the lengths of the cables connecting the load cells to the junction box. This is done in part because the cable is quite expensive. More importantly, a considerable loss of signal is experienced as the length of the cables increase. Any signal loss greatly reduces the accuracy of the scale system.
A typical prior art junction box distributes a 10 volt excitation power to each of the load cells. The excitation power is provided by a power supply which is often a part of the scale display. The junction box receives a signal output from each load cell. Through the application of electrical resistors, it sums and averages the signals. The averaged signal is then transmitted to the weight indicator. It will be readily appreciated that accurate calibration of each load cell is of utmost importance, because the resultant average analog signal is fed directly to the weight indicator.
Thus, in a properly calibrated four input system having four 1000 pound load cells at full capacity, each load cell would output a 30 millivolt signal. The junction box would receive, sum, and average the signals and would output the resultant average voltage. This average voltage would result from the 30 millivolt output from each load cell multiplied by 4 load cells for a total output of 120 millivolts which would be divided by 4, again the number of load cells, for a resultant average of 30 millivolts. This 30 millivolt signal would then be transmitted to the digital weight indicator which would be calibrated to read 4000 pounds with an input of 30 millivolts.
To illustrate the effect of a defective or uncalibrated load cell, let us assume that one cell failed completely. In this case the total input to the junction box would fall from 120 to 90 millivolts which when averaged by the number of load cells (4) results in an average of 22.5 rather than 30 millivolts. The 22.5 millivolts when transmitted to the digital weight indicator will result in a reading of 3000 pounds rather than the actual weight of 4000 pounds, a resultant error of 25 percent.
Once the load cell signals have been summed, a digital indicator cannot indicate that one or more load cells have failed or drifted out of calibration and that the displayed weight is incorrect. As indicated in the example set forth above, digital type indicators which are universally used, are calibrated over a range of zero to full span capacity. The primary function of the indicator is to convert from an analog millivolt signal to a scaled digital display shown as units of weight. Some digital indicators have the capability to detect a significant zero shift, but only when the weight vessel returns to a true zero.
The calibration of such prior art scales is quite involved For example, tanks and hopper scales, because of their size and design limitations, are extremely difficult to calibrate precisely during initial installation. Periodic recalibration or accuracy verification of such industrial type scales is even more costly, difficult and time consuming, because of the interruption of service and the requirement that the scales be unloaded. Therefore, a great many electronic scales are never retested for accuracy unless they fail completely. However, scales licensed by law must normally be tested at least once a year. In many applications, such as the weighing of explosives or other dangerous chemicals, frequent calibration is critical, since the average life expectancy of a load cell is only about six years. The major causes of load cell failure are shock loading, moisture, vibration, and heat. Moisture often enters a load cell through a cracked or deteriorated cable and shorts out the load cell.
Prior art scales of the type described above have no alarm means for notifying the user that a load cell has failed. Thus, without the knowledge of the scale operator a load cell may fail causing the digital indicator to be off by 25 percent. Accordingly, a scale which should be reading 100,000 pounds may be reading only 75,000 pounds and the scale operator would have no way of knowing that the scale was in error. It follows that the prior art devices provide no means for the scale user to quickly and easily verify the operability or calibration of the scale. In such prior art scales, operability or calibration tests require specialized electronic equipment not normally owned by scale users. Therefore, verification of an industrial scale's accuracy has been a procedure which could be accomplished only by scale specialist. Calibration verification of these scales is quite complicated, costly and time consuming. When possible calibration of industrial scales is done on weekends or other off-production times so as to minimize interference with production.